SPIDERS: CHARACTERISTICS, HABITAT, REPRODUCTION AND FEEDING - BIOLOGY - 2025

The spiders are a group of organisms belonging to the order Araneae. They are mainly characterized by having articulated appendages, distributed in two pairs of chelicerae, two pairs of pedipalps and four pairs of legs.

This order was first described in 1757 by the Swedish naturalist Carl Clerck. The order Araneae is currently considered to be the one with the largest number of species of all arachnids. They can be found in almost all terrestrial ecosystems.

Spider specimen. Source: Pixabay.com

Spiders are organisms with very interesting physical characteristics and behavior patterns. Due to this, more and more specialists are dedicated to its study, in an effort to fully elucidate its secrets.

Taxonomy

The taxonomic classification of spiders is as follows:

Domain: Eukarya

Animalia Kingdom

Phylum: Arthropoda

Subphylum: Chelicerata

Class: Arachnida

Order: Araneae

characteristics

Spiders are considered multicellular eukaryotic organisms, for two reasons. First, its genetic material (DNA) is located in a delimited structure within the cell known as the cell nucleus. Likewise, spiders are not made up of a single type of cells, but these have diversified and acquired various functions.

Taking into account the embryonic development of spiders, it can be safely stated that they are triblastic and protostome organisms. This means that they present the three germ layers: ectoderm, mesoderm and endoderm. In addition, from a structure known as the blastopore, both the anus and the mouth are formed simultaneously.

Spiders are made up of two exactly equal halves, which gives them bilateral symmetry. In turn, the vast majority of spider species have venom synthesizing glands, which they use mainly to capture and paralyze possible prey.

Spiders have the peculiarity of synthesizing a kind of thread, usually known as silk in many countries. This is nothing more than keratin (protein) that is subjected to a transformation process that gives it resistance and elasticity.

The usefulness of silk for spiders is very broad, and can be used to protect their eggs, paralyze prey and to cover their burrows, among other uses.

Spiders are carnivorous animals, which reproduce sexually through internal fertilization. They are also oviparous with indirect development.

Morphology

As with all members of the arthropod phylum, the body of spiders is divided into two segments or tagmas: the cephalothorax (prosoma) and the abdomen (opistosoma).

The size of the spiders is variable, depending on the different species that exist, and there may be spiders so small whose size does not exceed 5mm and spiders so large that they can measure up to more than 15 cm.

In the same way, spiders also have the representative element of arthropods: the articulated appendages. In spiders, the number of appendages is 12, distributed in pairs. The first of them corresponds to the chelicerae, the second to the pedipalps and the last four pairs are the legs of the animal.

- Cephalothorax (Prosoma)

It is the smaller portion of the two that make up the body of the animal. Its dorsal face is protected by a sclerosed plate that is convex known as the prosomic shield. On this surface are the organs of sight, made up of about eight eyes that are distributed in two parallel transverse lines.

The ventral part of the prosoma is occupied entirely by the sternum and the coxas of the legs. It is important to note that chelicerae are located towards the anterior part of the cephalothorax, at the base of which the animal's mouth opens.

Quéliceros

As in the rest of the chelicerates, the chelicerae constitute the first pair of appendages. In the case of spiders, they are small in size and at the distal end have a kind of nail. Depending on the species, these may or may not be associated with venom-synthesizing glands.

Pedipalps

The pedipalps of spiders are shorter than those of other arachnids, in addition to presenting a morphology similar to that of the legs. However, their function is not related to locomotion, but rather they have a sensory function.

In turn, pedipalps can fulfill other functions related to reproduction, especially in males.

Males can use pedipalps for the reproduction process, either for a courtship ritual, or as a copulatory organ to introduce the spermatophore into the female's body.

The morphology of the pedipalps will vary according to the function they fulfill and the species to which they belong.

Legs

Spiders have a total of eight legs, distributed in pairs. These articulate with the cephalothorax through its first joint, the coxa. Apart from this, they are made up of six other joints, from medial to lateral: trochanter, femur, patella, tibia, metatarsal and tarsus.

Depending on the species, it is possible that the legs have two or three claws at the tarsal level.

- Abdomen (Opistosoma)

It is generally bulky and globose in shape. Anatomically it is the place where the different systems that make up the animal are found, as well as some attached organs. The latter are important in the different functions that the animal may perform.

At the superficial level, the opistosome has several holes. Among these are the spiracles, which are the orifices into which the respiratory passages open. Another hole is the epiginium, the genital pore through which the fertilization process can take place.

Finally, it has an organ called spinerettes, generally six in number, organized in pairs. These are related to the production of silk.

- Internal anatomy

Digestive system

The digestive system of members of the order Araneae is of the complete type. It has two openings, one for the entrance or mouth and another for the exit known as the anus.

The mouth opens into the oral cavity in which a series of digestive enzymes are synthesized that contribute to the digestion of food.

Immediately after the oral cavity is the esophageal duct, which is short in length. The latter communicates with a wide cavity, the stomach. Here also other digestive enzymes are synthesized and secreted.

The stomach continues with the so-called midgut, which has a considerable length and is where the absorption process takes place. This intestine has sac-like structures called cecum. Their function is to increase the absorption surface.

Finally, there is the rectal blister that empties into the anus, which is where the waste substances produced by the digestive process are released.

Nervous system

The nervous system of spiders is made up of a series of ganglion groups that are distributed throughout the body of the animal.

At the prosome level, there is a ganglionic grouping that acts as the brain. This sends nerve endings to the numerous eyes (8) found in the prosoma.

Likewise, throughout the body of the spider there are some ganglia that emit nerve fibers mainly to the organs of the digestive system.

Internal anatomy of a spider. Source: Original: John Henry Comstock Vector: Pbroks13 (Ryan Wilson)

Circulatory system

Spiders have an open or lagoon type circulatory system. The main organ is a heart, which has several ostioli. The number of these depends on the evolutionary level of the spider species. This is how there are species that have hearts with two pairs of ostioles and others that have up to five pairs. The heart pumps the hemolymph throughout the body.

An anterior aorta and a posterior aorta emerge from the heart that expand their branches throughout the body of the animal, efficiently distributing the hemolymph, which is the fluid that circulates in this type of animal.

Excretory system

The main organs of the spiders' excretory system are the so-called Malpighi tubes, which are branched off the cecum of the midgut. These structures flow into the final portion of the digestive tract.

As with other arthropods, spiders have glands that lead to the coxas of the appendages. The most primitive species of spiders have two pairs of hip glands on the first and third pair of legs, while the more evolved species only have the hip glands of the first pair of legs.

Respiratory system

The respiratory system of spiders is similar to that of other arachnids, being made up of organs called lungs in books. These are constituted by invaginations of a tegumentary nature in which gas exchange takes place. Spiders can have one or two pairs of these structures.

The book lungs communicate with the outside through tubes that are called spiracles. Through these is that the air enters the body of the animal, carrying the oxygen to the lungs in book and excreting the carbon dioxide product of the gas exchange.

Reproductive system

Spiders are dioecious individuals, which means that the sexes are separated, that is, there are male and female specimens.

In the case of females, the reproductive system is represented by a pair of ovaries that can be shaped like a bunch of grapes, which contains the mature oocytes.

An oviduct arises from each ovary. These join in the midline of the animal's body, forming a single duct, which empties into the vagina, whose hole is in the middle portion of the so-called epigastric fold. Likewise, they have an opening called the epiginium, which communicates with a storage organ called the spermatheca.

In the case of male individuals, the reproductive system is made up of two testes that communicate with the outside world through a genital pore in the opistosome. They also have copulatory organs, which are located in the pedipalps of the animal.

Habitat and distribution

Spiders constitute one of the most widely distributed groups of animals throughout the universal geography. They have managed to conquer all habitats, with the exception of the Antarctic continent.

In general, depending on the ecosystem in which they are found, spiders have had to undergo certain changes in order to adapt to these.

For example, in the case of spiders found in desert ecosystems, they have developed mechanisms to take advantage of the water present in the prey they ingest and thus not require an external water source.

Classification

The order Araneae is made up of three suborders: Mesothelae, Mygalomorphae and Araneomorphae.

Mesothelae

They are characterized because they do not have venom synthesizing glands, in addition to having a fairly narrow sternum compared to other types of spiders. It is made up of three families, of which two are considered extinct. The only one that has survived to date is Liphistiidae.

Mygalomorpheae

Tarantula. Source: pixabay.com

These spiders are characterized by being large and quite robust. They have poisonous glands whose ducts are found inside strong and powerful chelicerae. A representative specimen of this suborder is the tarantula.

Araneamorphae

It is the suborder that covers the largest number of species, which are grouped into a total of 92 families. Its distinctive element is the diagonal chelicerae, which intersect at their distal ends.

Feeding

Spiders are carnivorous predatory animals that have some highly effective mechanisms in capturing prey.

When the spider identifies a potential prey, they can capture it using the silk webs it produces. Once the prey is trapped in the web, the spider inoculates its venom with its chelicerae.

This poison causes the prey to become paralyzed, which allows the spider to inject digestive enzymes to initiate their action. Digestive enzymes degrade the prey and transform it into a kind of porridge, which is ingested by the animal.

Inside the animal's body, the food passes into the stomach, where it continues to suffer the action of the digestive enzymes synthesized there. Later it passes to the intestine where the absorption process takes place. Substances that are not used by the animal's body are excreted through the anus.

Reproduction

Spiders reproduce through sexual mechanisms. This involves the fusion of male and female gametes. Fertilization is internal and indirect, that is, it occurs inside the female's body, but it does not involve a copulation process.

The reproduction process of spiders is one of the most complex in the animal kingdom since it includes mating rituals. In the first place, the female is capable of releasing chemical substances called pheromones, which constitute chemical signaling agents that attract the male to the beginning of the reproductive process.

Likewise, there are species in which the male performs a kind of dance whose purpose is to be noticed by the female and induce the reproduction process.

Later the male releases a spermatophore in which the sperm are contained. Then, with the help of their pedipalps, the spermatophore is introduced into the female so that the fertilization process finally occurs.

Taking into account that spiders are oviparous organisms, after fertilization the female lays the eggs. These measure approximately 2 mm and embryonic development lasts between 1 month and 1 month and a half.

Spider eggs Source: jenis Patel At the end of that time, the eggs hatch and individuals emerge that have the same characteristics of an adult, but smaller. In the course of time, the spider undergoes several molts until reaching adulthood and sexual maturity.

As a curious fact, at the end of fertilization, some female spiders usually devour the male.

Representative species

Latrodectus mactans

Also known as "black widow", they belong to the suborder Araneomorphae, being a fairly well-known species of spider, especially due to the toxicity of its venom. It is easily identifiable thanks to a red hourglass-shaped spot on the lower end of its abdomen.

Lycosa tarantula

It belongs to the suborder Araneomorphae. It is a large spider, of which specimens that reach up to 30 cm have been found, including the length of its appendages.

They have a fearsome appearance and are very famous for the dangerousness of their poison. In humans, although it is not fatal, its toxin can cause necrosis in muscle tissue.

Theraphosa blondi

It is the so-called "Goliath tarantula." It is one of the most feared spiders due to its imposing appearance. Likewise, it is considered the heaviest in the world, reaching in some cases up to 170 grams. Its entire body is covered with hair and it also has wide and robust pedipalps.

Specimen of Theraphosa blondi (Goliath Tarantula). Source: Www.universoaracnido.com

Gold silk spider

They are a group of spiders that belong to the genus Nephila. They are characterized by the bright colors they exhibit, among which yellow and ocher tones can be mentioned. Likewise, they owe their name to the color of the thread with which they weave their web.

References

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